Various types of sewing machines provided with a needle threading mechanism capable of automatically threading an eye of the sewing needle have been conventionally suggested. For example, a sewing machine disclosed in JP 2004-222916 A (hereinafter referred to as patent document 1) is provided with a thread drawing mechanism including a drive motor, a sector gear, and a thread drawing member; and a needle threading mechanism including a needle threading motor and a needle threading hook. In such sewing machine, first, the needle thread is manually threaded on a hook portion of a needle threading hook which has passed through the eye of the sewing needle when the needle threading hook is driven into a threading position from a stand-by position. Then, when the needle threading hook is retracted to a loop cancel position, the thread drawing member is driven into the thread drawing position from the stand-by position to be engaged with the thread end side of the needle thread. Thereafter, the thread drawing member returns to the stand-by position and the needle thread end is retained by a first thread retaining portion.
In the sewing machine described in patent document 1, the needle threading hook is threaded as follows. The needle thread drawn from the thread spool is manually engaged with the hook portion of the needle threading hook after being threaded to a thread engagement portion of a check spring and the thread-take up; and thereafter, the needle thread end is retained by the second thread retaining member provided in a needle bar case. At this time, thread tension is exerted on the needle thread depending upon the manner of threading operation and the thread engagement portion of the check spring may be rotated to a spring force operating position by the thread tension.
When the needle threading hook is retracted from the threading position in which the needle thread is threaded thereto to the loop cancel position in which the needle threading hook comes out of the eye of the sewing needle, the needle thread end is gradually pulled out of the second thread retaining portion. When the needle thread end is completely pulled out of the second thread retaining portion, needle thread tension is cancelled so that the engagement portion of the check spring is rotated to an inoperable position by the spring force. At this time, the needle thread in the thread end side is loosened by the cancellation of the needle thread tension and at the same time is pulled at once toward the check spring by the returning force of the check spring, thereby exhibiting instability in operation, leading to needle threading failures where the needle thread is removed from the needle threading hook.